Method of making arsenates



Patented June 15, 1926.

UNITED STATES PATENT OFFICE.

may 2. nassnr'r, or cnz'rnxmgxnn'rucxx, am) nmsna'r c. LATHROP, or

PHILADELPHIA,

METHOD MAKING ABSEN'ATES.

10 Drawing.

This invention relates to methods of oxidizing arsenious oxide, arsenious acid, and its compounds, and. more particularly to methods of oxidizing these materials in which chlorine is employed.

This invention is an improvement over the process disclosed in the copending ap-' plication of Harry P. Bassett, filed Febru- 111') 19, 1923, SerialNo. 620,050, for methods of making arsenates.

All of the arsenate in commerce today is produced by the oxidation of arsenious acid with concentrated nitric acid, the material produced being then neutralized with the desired base. A serious objection to this method is that nitric acid is not complete in its oxidizingpower and is not even in its action. ThlS often causes explosions, due to sudden reactions taking place. Nitric acid does not act evenlyon arsenious oxide unless the oxide is of 99 per cent purity or better.

This causes a limit on the amount of arsenic available, and the lower arsenic containing compounds cannot be employed as a substitute therefor. The compounds of arsenious oxide, such as sodium and calcium arsenites, are not available for the production of arsenate's when the nitric acid method is used.

. Heretofore, it has been impossible to successffilly and commercially produce 'arsenates by means of chlorine.

In the present invention, we provide a method by means of which, the lower arsenic containing compounds, including the arsenites, may be oxldized by means of chlorlne. Thearsenious acid or one of its compounds is treated with chlorine or bleaching powder 40 in the presence of an alkali metal salt or a metallic salt which agents seem to have a catalytic eflect on the oxidation. A solution of white arsenic is first prepared and bleaching powder is added thereto in the 4-6 theoretical amount, the reaction being permitted to take place in a closed vessel, giving the arsenious oxide timeto react with the chloride of lime (calcium hypochlorite) present to form a mixture of monocalcium arsenate and dicalcium arsenate.

The arsenate is treated with enou h lime to set the arsenic, and not form bas1c calc um chloride, forming a mixture of dlcalclum Application filed April 10,1924. Serial in. 705,693.

metal, capable of forming an-insoluble calcium salt with the calcium cations of the arsenates is added, to precipitate all of the calcium, and leaving a soluble arsenate under proper conditions of temperature. In

practice, we prefer to use sodium carbonate. The calclum carbonate or other salt of calcium is filtered oil, and the sodium arsenates and other sodium salts which remain in the filtrate are digested with an insoluble lead compound, such as lead monoxide -(litharge), capable of forming lead arsenate. This is filtered off, washed and dried The filtrate and washing are eva orated back to volume. gases-or other sources of carbon dioxide, and

reused to form some impure sodium -ar senates again.

The lead or sodium salts formed in the two above steps of this process may be the di or tri salts, but this matter of exact ratio of lead or sodium to the arsenates formed is not material to this invention. It is obvious that other arsenates, such as copper, may be prepared in a similar way. J

In a typical embodiment of the invention, a solution or suspension of white arsenic or one of its compounds is first prepared and bleaching powder is added thereto in the theoretical amount. The mixture is placed in a closed vessel where any chlorine that is liberated is prevented from escaping, giving the arsenious oxide time to react with the lime and lime salts present. The reaction forms a mixture of mono-calcium and di-calcium arsenate along with the formation of calcium chloride, which acts as a catalyzer during the reaction.

If gaseous chlorine is used, some lime must be added to the mixture before the chlorine will react efiiciently with the arsenious oxide. The mixture of mono-calcium and dicalcium arsenate resulting from the oxidation is dumped into an agitating tank and enough lime added to set the arsenic. By setting is meant the conversion of the mixture of mono-calcium arsenate and dicalcium arscnate to di-calcium arsenateand tri-calcium arsenate, which are insoluble.

This liquor is car onated by flue During this part of the process, it is advisable to heat the solution to a temperature of substantially 60 0., which promotes the reaction. The slurry formed is then fed to a 'filter, and the arsenate filtered 013" and washed to remove the calcium chloride.

The press cake containing the insoluble calcium arsenates is then heated or boiled in water containing the requisite amount of soda ash to convert all the calcium to calcium" carbonate, thus producing a mixture of sodium arsenates in solution.

To obtain the sodium arsenate, we either evaporate or concentrate this aqueous solution without special treatment, or else add sodium hydroxide to produce substantially tri-sodium arsenate. v

The sofor1ned arsenate or sodium arsenates are digested with lead monoxide (litharge) in the preferred embodiment of our invention to produce lead arsenate. Insoluble lead arsenate and sodium hydroxide are formed. The insoluble lead arsenate is fil-' tered and washed. The filtrate and washings contain sodium hydroxide and the excess sodium arsenate iised in the conversion reaction, and after carbonating with carbon dioxide, the mixed sodium carbonate and sodium arsenate is used to react on calcium arsenates. As the presence of a little sodium hydroxid or sodium plumite promotes the reaction of lead monoxide upon sodium arsenates, we prefer to leave a small quantity of sodium hydroxide uncarbonated. This precipitate is filtered, washed, and dried.

While we have described in detail the preferred practice of our rocess, it is to be understood that the detai s of procedure, and

pro ortion of ingredients may be widely va-- rie and that known chemical equivalents may be employed in place of the materials mentioned without departing from the spirit of the invention or the scope of the subjoined claims.

We claim:

1. The method of oxidizing arsenious compounds to form arsenates which comprises treating said compounds with chlorine in the presence of a base capable of forming a soluble chlorid and a substantially insoluble arsenate, andthen treating this substantially insoluble arsenate with a compound, the cation of which forms a more soluble metal arsenate or mixtures of more soluble arsenates and the anion of which combines with the base of the substantially insoluble arsenate to form a more insoluble compound.

'2. The method of oxidizing arsenious compounds to form arsenates which comprises treating said compounds with chlorine in the presence of a base capable of forming a solu le chlorid and a substantially insoluble arsenate, and then treating this substantially insoluble arsenate with a compound, the cation of which forms a soluble alkali metal arsenate or mixture of soluble alkali metal arsenates and the anion of which combines with the base of the substantially insoluble arsenate to form a more insolublecompound.

3. The method of oxidizing arsenious com-- poundsto form arsenates which comprises treating said compounds with chlorine in the presence of a base capable of forming a solub-le chlorid and a substantially insoluble arsenate, and then treating this substantially insoluble arsenate with a compound, the cation of which forms sodium arsenate or mixtures of sodium arsenates'and the anion of which combines with the base of the substantially insoluble arsenate to form a more insoluble compound. a

4, The method of oxidizing arsenious compounds to form arsenates which comprises treating said compounds with chlorine in the presence of lime and then treating the calcium arsenate formed with a compound,

the cation of which forms sodium arsenate, or mixture of sodium arsenates, and the anion of which combines with the calcium to form a more insoluble calciumzcompound.

5. The method of oxidizing arsenious compounds to form arsenates which comprises treating said compounds with chlorine in the presence of lime, se arating the calcium arsenates from the ca cium chlorid formed by filtration and then treating the calcium arsenate with a compound, the cation of which forms a more soluble metal arsenate or mixture of more soluble arsenates and the anion I of which combines with the calcium to form a more insoluble calcium compound.

6. The method of oxidizing arsenious com-' pounds to form arsenates which comprises treating said compounds with chlorine in the presence of lime, then separating the calcium arsenates from the calcium chlorid formed by filtration and then treating the calcium arsenates with sodium carbonate to form sodium arsenate or mixture of sodium arsenates and calcium carbonate, and filtering off the calcium carbonate.

7. The method of oxidizing arsenious compounds to form arsenates which comprises treating said compounds with chlorine in' the presence of lime to form calcium. chlorid and a substantially insoluble arsenate, then treatin this substantially insoluble arsenate with a compound, the cation of which forms sodium arsenate or mixture of sodium arsenates, and the anion of which combines with the base of the substantially insoluble arsenate to form a more insoluble compound, and then treating the arsenates with an alkali metal hydroxid to form sodium tri-arscnate.

'8. The method of oxidizing arsenious com-' pounds to form arsenates which comprises treating said compounds with; chlorine in the presence of lime to form calcium chlorid and a substantially-insoluble arsenate, and- 1,asa,eo1, I F8 then treatin this substantially insoluble from the insoluble compound and adding a arsenate' wit a compound, the .cation of further quantity of sodium compound to the which forms sodium arsenate or mixture of arsenates to form tri-arsenate. 10 sodium arsenates and the anion of .which In testimony'whereof we aflix our signs- I combines with the base of the substantially tures. insoluble arsenate to form a more insoluble HARRY P. BASSETT. compound, separating the sodium arsenates ELBERT G. LATHROP. 

